<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>5-元素整体透明度</title>
    <style>
        div{
            width: 300px;
            height: 300px;
            background: red;
            text-align: center;
            opacity: .7;
        }
    </style>
</head>
<body>
    <!-- 
        整体透明度:  [0,1]
            opacity:.7
     -->
    
     <div>
        <img src="images/200.png" alt="">
     </div>

</body>
</html>